Radio frequency (RF) power amplifiers are often used in the front end circuitry of a mobile device for amplifying baseband signals to appropriate levels for transmission from an antenna. Due to the evolution of wireless communication standards as well as the demand for smaller, more efficient mobile devices, the complexity of RF power amplifiers continues to increase. Modern front end circuitry may include RF power amplifiers with multiple gain stages in order to increase the efficiency of a mobile device. Each gain stage may be configured to transmit a signal at a given power. While the use of multi-stage RF power amplifiers has a positive impact on the efficiency of front end circuitry in which it is incorporated, the multi-stage approach often results in a non-continuous gain response of an RF power amplifier. Discontinuities in the gain response of an RF power amplifier may cause the RF power amplifier to fail to meet one or more standards requirements, such that the device may not be incorporated into a mobile device suitable for distribution or sale.
FIG. 1 shows a conventional transmit chain 10 for a mobile terminal. The conventional transmit chain 10 includes transceiver circuitry 12, one or more RF power amplifiers 14A-14N (referred to collectively as the RF power amplifiers 14), antenna switching circuitry 16, control circuitry 18, and an antenna 20. Each one of the RF power amplifiers 14 is coupled between the transceiver circuitry 12 and the antenna switching circuitry 16. The antenna 20 is coupled to the antenna switching circuitry 16, such that the antenna switching circuitry 16 is used to select one of the RF power amplifiers 14 to couple to the antenna 20. The control circuitry 18 is coupled to the transceiver circuitry 12, each one of the RF power amplifiers 14, and the antenna switching circuitry 16.
In operation, the transceiver circuitry 12 receives digitized data, which may represent voice, data, or control information. The digitized data is modulated to produce a carrier signal at a desired transmit frequency. The carrier signal is then delivered to one or more of the RF power amplifiers 14, where it is amplified and delivered to the antenna switching circuitry 16. The antenna switching circuitry 16 selectively couples one or more output terminals of the RF power amplifiers 14 to the antenna 20 in order to transmit the carrier signal. The control circuitry 18 may control one or more operating parameters of the transceiver circuitry 12, the RF power amplifiers 14, and the antenna switching circuitry 16.
FIG. 2 shows details of the first RF power amplifier 14A in the conventional transmit chain 10. For context, the control circuitry 18 is also shown. The first RF power amplifier 14A is a multi-stage RF power amplifier including a first gain stage 22, a second gain stage 24, and a third gain stage 26. The first gain stage 22, the second gain stage 24, and the third gain stage 26 are bipolar junction transistors (BJTs) including a collector terminal C, an emitter terminal E, and a base terminal B. The base terminal B of the first gain stage 22 is coupled to an input terminal 28 through a first coupling capacitor 30. The collector terminal C of the first gain stage 22 is coupled to the base terminal B of the second gain stage 24 through a second coupling capacitor 32. The emitter terminal E of the first gain stage 22 is coupled to ground.
The collector terminal C of the second gain stage 24 is coupled to the base terminal B of the third gain stage 26 through a third coupling capacitor 34, and also to a low-power amplifier path 36 through a fourth coupling capacitor 38. The emitter terminal E of the second gain stage 24 is coupled to ground. The collector terminal C of the third gain stage 26 is coupled to a high-power amplifier path 40. A supply voltage VDD is provided to the collector terminal C of each one of the first gain stage 22, the second gain stage 24, and the third gain stage 26 through a first choke inductor 42A, a second choke inductor 42B, and a third choke inductor 42C, respectively. First gain stage bias circuitry 44 is coupled to the base terminal B of the first gain stage 22 and the second gain stage 24. Second gain stage bias circuitry 46 is coupled to the base terminal B of the third gain stage 26. The low-power amplifier path 36 includes low-power matching circuitry 48. The high-power amplifier path 40 includes high-power matching circuitry 50. The control circuitry 18 is coupled to the first gain stage bias circuitry 44, the second gain stage bias circuitry 46, and the low-power matching circuitry 48.
In operation, the first RF power amplifier 14A receives a signal at the input terminal 28 from the transceiver circuitry 12 (FIG. 1). The signal is passed through the first coupling capacitor 30 to the base terminal B of the first gain stage 22, where it is amplified and presented at the collector terminal C of the first gain stage 22. The amplified signal is then passed through the second coupling capacitor 32 to the base terminal B of the second gain stage 24, where it is further amplified and presented at the collector terminal C of the second gain stage 24. When the first RF power amplifier 14A is in a low-power mode of operation, the amplified signal is delivered to the low-power amplifier path 36 through the fourth coupling capacitor 38. The signal is then passed through a low-power bypass switch 52 to the low-power matching circuitry 48, and subsequently delivered to an output terminal 54 through a low-power output switch 56. When the first RF power amplifier 14A is in a high-power mode of operation, the amplified signal is delivered to the base terminal B of the third gain stage 26, where it is further amplified and presented at the collector terminal C of the third gain stage 26. The amplified signal is then delivered to the output terminal 54 through the high-power matching circuitry 50 and a high-power output switch 58 in the high-power amplifier path 40.
The control circuitry 18 may control one or more operating parameters of the first gain stage bias circuitry 44, the second gain stage bias circuitry 46, the low-power bypass switch 52, the low-power output switch 56, and the high-power output switch 58. In the low-power mode of operation of the first RF power amplifier 14A, the control circuitry 18 may operate the first gain stage bias circuitry 44 such that the first gain stage 22 and the second gain stage 24 are placed in an active, or ON state, and operate the second gain stage bias circuitry 46 such that the third gain stage 26 is placed in an OFF state. Further, in the low-power mode of operation, the control circuitry 18 may close the low-power bypass switch 52, close the low power output switch 56, and open the high-power output switch 58 in order to direct the amplified signal to the output terminal 54 through the low-power amplifier path 36.
In the high-power mode of operation of the first RF power amplifier 14A, the control circuitry 18 may operate the first gain stage bias circuitry 44 and the second gain stage bias circuitry 46 such that the first gain stage 22, the second gain stage 24, and the third gain stage 26 are placed in an active, or ON state. Further, in the high-power mode of operation, the control circuitry 18 may open the low-power bypass switch 52 and the low-power output switch 56 while closing the high-power output switch 58 to direct the amplified signal to the output terminal 54 through the high-power amplifier path 40.
The low-power matching circuitry 48 and the high-power matching circuitry 50 may include active or passive components adapted to match an impedance at the output of the collector terminal C of the second gain stage 24 and the third gain stage 26, respectively, to an impedance presented at the output terminal 54.
Depending on the desired output power of a mobile terminal in which the first RF power amplifier 14A is incorporated, either the low-power mode of operation or the high-power mode operation of the first RF power amplifier 14A is chosen. In order to comply with one or more mobile standards, a fixed gain delta is desired between low-power mode and high-power mode.
In either the low-power mode of operation or the high-power of operation, the load presented at the antenna 54 may deviate from the designed load of 50Ω, thereby changing the overall gain of RF power amplifier 14A. As will be appreciated by those of ordinary skill in the art, when the RF power amplifier 14A is switched between its respective modes of operation, the load lines of the second gain stage 24 operating in the low-power mode and the third gain stage 26 operating in the high-power mode will differ, resulting in a non-continuous gain response of the first RF power amplifier 14A at the switching point of the low-power mode of operation and the high-power mode of operation. This non-continuous gain response may cause the first RF power amplifier 14A to fail to meet standards requirements across a full dynamic range of desired output power at the vicinity of the low-power mode and high-power mode switching point, such that the device cannot be incorporated into a mobile terminal suitable for distribution or sale.
FIG. 3 shows a chart depicting the non-continuous gain response of the first RF power amplifier 14A. The gain of the first RF power amplifier 14A in the high-power mode of operation with respect to the phase of a load attached to the output terminal 54 is shown by a solid line 60. The gain of the first RF power amplifier 14A in the low-power mode of operation with respect to the phase of a load attached to the output terminal 54 is shown by a dashed line 62. Finally, the gain delta, or difference in the gain response of the first RF power amplifier 14A in the high-power mode of operation and the low-power mode of operation, is shown by a semi-solid line 64. As seen in FIG. 3, the gain response of the first RF power amplifier 14A in the high-power mode of operation is shifted in phase from the gain response of the first RF power amplifier 14A in the low-power mode of operation, causing a maximum gain delta of around 1.5 dB. Accordingly, switching between the high-power mode of operation and low-power mode of operation will cause a non-continuous gain response of the first RF power amplifier 14A.
As discussed above, discontinuities in the gain response of the first RF power amplifier 14A may cause the first RF power amplifier 14A to fail to meet standards requirements, such that the device cannot be incorporated into a mobile terminal suitable for distribution or sale. Accordingly, multi-stage RF amplifier circuitry is needed that is capable of producing a continuous gain response over a variety of load phase conditions.